if by cache you mean that your entire system is on your drive and that the hdd is for backups or games that you’re not currently playing but don’t want to reinstall, yes.

it’s not secure by design, since it’s not made to be secure, and also uses unstable versions of a lot of packages to make certain exploits work

yes

android is very sandboxed, so anything you do in termux/userland can’t affect the android on your phone (unless you’re rooted)